Istanbul, the economic capital and most populated city of Turkey, is highly prone to earthquakes. When an earthquake occurs, required relief items are expected to be supplied from national and international sources. To alleviate human suffering following an earthquake, in this paper, we propose an intermodal relief item distribution model for Istanbul involving sea and land transportation with identified road vulnerabilities. The proposed mathematical model utilizes the seaports of Istanbul for maritime transportation and allows for the transportation of relief item between Istanbul's European and Anatolian sides. We also use the seabasing concept for providing supplies to demand areas. The use of maritime transportation and seabasing provides flexibility for humanitarian logistical activities and the proposed model provides an effective and reliable disaster relief system for Istanbul as well as other disaster-prone cities with significant maritime transportation components.